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MICROBIALITE FORMATION IN SEAWATER OF INCREASED ALKALINITY, SATONDA CRATER LAKE, INDONESIA GERNOTARP,ANDREASREIMER,ANDJOACHIMREITNER GeowissenschaftlichesZentrumderUniversita¨tGo¨ttingen,AbteilungGeobiologie,Goldschmidtstraße3,D-37077Go¨ttingen,Germany e-mail:[email protected] ABSTRACT: The crater lake of the small volcanic island Satonda, In- 1986) but also in its entire major ion composition (Spencer and Hardie donesia,isuniqueforitsred-algalmicrobialreefsthrivinginmarine- 1990;Hardie1996;Arpetal.2001;Lowensteinetal.2001). derivedwaterofincreasedalkalinity.Thelakeisapotentialanalogue Nonetheless, the significance of absolute ion concentrations,alkalinity, for ancient oceans sustaining microbialites under open-marinecondi- and CaCO supersaturation in microbialite formation remains a matter of 3 tions.Currentreefsurfacesaredominatedbylivingredalgaecovered discussion(e.g.,KempeandKazmierczak1990a,1990b,1994;Grotzinger bynon-calcifiedbiofilmswithscatteredcyanobacteriaanddiatoms.Mi- 1990; Knoll et al. 1993; Grotzinger and Knoll 1995). This is especially nor CaCO precipitates are restricted to the seasonally flooded reef trueforCa2(cid:49)andalkalinitybecausebotharestronglyaffectedbybiological 3 tops, which develop biofilms up to 500 (cid:109)m thick dominated by the processes,suchasactiveCa2(cid:49)removalbycellularionpumpsandHCO (cid:50) 3 cyanobacteriaPleurocapsa,Calothrix,Phormidium,andHyella.Micro- releasefrombacterialsulfatereduction.Weatheringandplatetectonicpro- crystalline aragonite patches form within the biofilm mucilage, and cesses are two additional factorsaffecting seawatercomposition.Further- fibrousaragonitecementsgrowinexopolymer-poorspacessuchasthe more,changesofpCO intheatmosphere(seeRoyeretal.2001forreview) 2 inside of dead, lysed green algal cells,andreef frameworkvoids.Ce- havetobetakenintoconsiderationwhendiscussingconstraintsonancient mentation of lysed hadromerid sponge resting bodies results in the seawater alkalinities and CaCO mineral supersaturation (e.g.,Mackenzie 3 formationof‘‘Wetheredella-like’’structures. andPigott1981;KempeandDegens1985;MackenzieandAgegian1989; Hydrochemistry data andmodel calculationsindicatethatCO de- Grotzinger 1990, 1994; Morse and Mackenzie 1998). Thus, in situ calci- 2 gassing after seasonal mixis can shift the carbonate equilibrium to fying biofilms and microbial mats forming in modified seawater are of cause CaCO precipitation. Increased concentrations of dissolved in- specialinterestaspotentialanaloguesforfossilmicrobialitesofopenma- 3 organiccarbonlimittheabilityofautotrophicbiofilmmicroorganisms rine settings. Knowledge of their formation processes may provide indi- to shift the carbonate equilibrium. Therefore,photosynthesis-induced cationsforthereconstructionoftheambientseawaterchemistry. cyanobacterialcalcificationdoesnotoccur.Instead,passive,diffusion- Aquasi-marinealkalinelakesustainingmicrobialiteformationhasbeen controlledEPS-mediatedpermineralizationofbiofilmmucusatcontact described from Satonda (Kempe and Kazmierczak 1990a, 1990b, 1993; with the considerably supersaturated open lake watertakesplace.In Kempeetal.1996,1997),asmallvolcanicislandnorthofSumbawa,In- contrasttoextremesodalakes,thereleaseofCa2(cid:49)fromaerobicdeg- donesia (Fig. 1). These microbialites form a part of the red-algal-micro- radationofextracellularpolymericsubstancesdoesnotsupportCaCO bialite reefs, which occur at protrusions of the rocky lake shore (Fig. 1). 3 precipitation in Satonda because the simultaneously released CO is Kazmierczak and Kempe (1990) suggested that the microbialites formed 2 insufficientlybuffered. bycalcifyingcyanobacterialmatsandcomparedthemwithPaleozoicstro- Subfossil reef parts comprise green algal tufts encrustedbymicro- matoporoids. They further argued that this similarity was a reason to be- stromatolites with layers of fibrousaragoniteand anamorphous,un- lievethatearlyPaleozoicseawaterhadahigheralkalinityandCaCO su- 3 identifiedMg–Siphase.Themicrostromatolitesprobablyformedwhen persaturationthanmodernseawater. LakeSatondaevolvedfromseawatertoCa2(cid:49)-depletedraised-alkalinity The purpose of this paper is to elucidate mechanisms of microbialite conditions because of sulfate reduction in bottom sedimentsandpro- formationinseawaterofincreasedalkalinity.Wefocusonprecipitationin nounced seasonality with deep mixing events and strong CO degas- recent biofilms in the lake in relation to biofilm structure and seasonal 2 sing.Thelattereffectcausedrapidgrowthoffibrousaragonite,while hydrologiccycleinSatonda.Investigationofmechanismsofrecentbiofilm Mg–Si layers replaced the initially Mg-calcite-impregnated biofilms. calcification serves as a basis for exploring the formation of the unique Thiscouldbeexplainedbydissolutionofsiliceousdiatomsandsponge subfossil microbialite. Finally, results of the present study are compared spicules at high pH, followed by Mg-calcite dissolution and Mg-silica andcontrastedwiththemodelofSatondaLakemicrobialiteformationpro- precipitation at low pH due to heterotrophic activity within the en- posed by Kazmierczak and Kempe (1990, 1992) and Kempe and Kaz- tombedbiofilms. mierczak(1990a,1990b,1993). FACTORS CONTROLLINGBIOFILM CALCIFICATION INTRODUCTION Biofilms consist of microbial cells, mainly prokaryotes of severalmet- Normal-marinesettingstodaysustainmicrobialiteformationonlyinex- abolicgroups(VanGemerden1993),embeddedinahighlyhydratedmu- ceptional cases.Thereisonly oneknown exampleof lithifyingstromato- cilagecomposedofextracellularpolymericsubstances(EPS)(Decho1990; lites (Bahamas) in an open marine setting of normal seawater salinity Wingenderetal.1999).Incontrasttobiomineralizationineukaryoticalgae (Dravis 1983; Dill et al. 1986; Reid et al. 2000; Visscher et al. 2000). andmetazoa(Westbroeketal.1984;AddadiandWeiner1985;Mann1988; However, many marine fossil microbialites differ from the agglutinated LowenstamandWeiner1989;SimkissandWilbur1989),precipitationin Bahamianstromatolitesinthattheycontainlesstrappedparticlesandsub- biofilmsisrarelycontrolledbythemicroorganismsandisregardedasin- stantially more in situ precipitated carbonate (Gebelein 1976; Grotzinger ducedormediated(e.g.,Pentecost1991;Riding1991a,2000).Indeed,non- 1990;Riding1991a,2000)—similartothestromatolitesofthenonmarine living organic matter can mineralize without apparent direct involvement typelocality(Kalkowsky1908;PaulandPeryt1999).Onereasonforthis of living cells, a process known as ‘‘organomineralization’’ (Trichet and discrepancy between recentandpre-Tertiarymarinecarbonatesedimenta- De´farge1995;De´fargeetal.1996;seealsoReitneretal.1995). tionisprobablyachangeinoceanchemistry.Seawaterchemistrychanged Therearethreemajorfactorsthataresignificantinbiofilmcalcification. through time not only with regard to the Mg/Ca ratio (Wilkinson 1979; First,istheinitialdissolvedinorganiccarbonpoolandsaturationstatewith Riding1982;Sandberg1983;Wilkinsonetal.1985;WilkinsonandGiven respecttoCaCO minerals.Approximatelyten-foldcalcitesupersaturation 3 JOURNALOFSEDIMENTARYRESEARCH,VOL.73,NO.1,JANUARY,2003,P.105–127 Copyright(cid:113)2003,SEPM(SocietyforSedimentaryGeology) 1527-1404/03/073-105/$03.00 106 G.ARPETAL. FIG. 1.—A) Location of the Satonda island, north of Sumbawa, Sunda archipelago, Indonesia. B) Bathymetric map of the Satonda Crater Lake (from Kempe and Kazmierczak1990,modified).Arrowsindicatereefsitessampledinthisstudy.C)Schematicsectionofthered-algal-microbialitereefs.Drawingisnottoscale. (i.e.,SI (cid:53)1.0;SI (cid:53)0.86;fordefinitionofSIseeTable1)seemsto low-DICsettings,whereashigh-DICsettingsarealmostunaffected(Arpet Cc Arag be a prerequisite for biofilm calcification (Arp et al. 1999a; Arp et al. al.2001). 1999b, 2001). This level of threshold supersaturation for CaCO precipi- The second major factor in biofilm calcification are physiological pro- 3 tationvariesindifferentsettings,mainlydependentontheMg2(cid:49),SO 2(cid:50), cesses of microorganisms; theseprocessescould alterthe carbonateequi- 4 andPO 3(cid:50)concentrations.Startingfromthelevelofinitialsupersaturation, librium and Ca2(cid:49) concentration in the microenvironment. Physiological 4 the effect of carbon fixation by organisms on the CaCO supersaturation processes capable of inducing CaCO precipitation (i.e.,temporarilyshift 3 3 depends on the concentration of dissolved inorganic carbon (DIC). The the SI to values higher than 1.0) are autotrophic CO fixation, nitrate Cc 2 same amount of fixed carbon causes a great change in supersaturationin reduction and ammonification, sulfate reduction, and coupled sulfate re- TABLE1.—WaterchemistryofsamplesfromthreedifferentwaterdepthsthatarerepresentativeofthethreelakewaterbodiesofSatondaCraterLake,respectively. TotalAlkb DICc Ca2(cid:49) Mg2(cid:49) pCOf Sample Depth T(cid:56)C pH p(cid:171)a Salinity‰ meqL(cid:50)1 mmolL(cid:50)1 mmolL(cid:50)1 mmolL(cid:50)1 SI d SI d (cid:109)atm2 Cc Arag SamplingperiodOctober1993 Mixolimnion(0–22m) 0.1m 30.7 8.58 4.52 31.4 4.17 3.41 4.64 42.50 1.00 0.86 282 5m 30.9 8.59 4.38 31.4 4.15 3.38 4.58 42.58 1.00 0.86 269 Monimolimnion,concentratedlayer(22–50m) 30m 29.8 7.28 (cid:50)1.92 37.3 7.38 7.74 5.53 49.88 0.19 0.05 14791 Monimolimnion,brine(50–69m) 60m 29.4 6.94 (cid:50)2.94 41.7 50.43 56.45 5.93 57.58 0.68 0.54 218776 SamplingperiodJune1996 Mixolimnion(0–24m) 0.5m 30.6 8.50 6.31 29.4 3.97 3.33 4.55 43.57 0.92 0.78 339 5m 30.5 8.58 6.31 29.4 4.04 3.30 4.65 43.28 0.99 0.85 269 Monimolimnion,concentratedlayer(24–51m) 30m 29.7 7.35 (cid:50)2.12 37.2 7.60 7.89 5.93 51.39 0.29 0.15 12589 Monimolimnion,brine(51–70m) 60m 28.9 6.97 (cid:50)3.54 41.6 47.56 52.87 6.30 58.25 0.71 0.57 186209 Standardseawaterb 25.0 8.22 8.45 35.0 2.406 2.18 10.66 55.07 0.76 0.61 417 aRedoxintensityp(cid:171)(cid:53)(cid:50)log{e(cid:50)} bTotalalkalinity(cid:53)acid-neutralizingcapacityexpressedasmilliequivalentperliter cDissolvedinorganiccarbonDIC(cid:53)[CO(aq)](cid:49)[HCO](cid:49)[HCO(cid:50)](cid:49)[CO2(cid:50)] dSI ,SI :Saturationindexforcalcite2andaragonit2e;S3I(cid:53)log(IA3P/K),wher3eIAP(cid:53)ionactivityproductofCa2(cid:49)andCO2(cid:54)s,andK(cid:53)solubilityproductofcalciteandaragonite,respectively. Cc Arag 3 efromNordtrometal.(1979). fPartialpressureofcarbondioxide. MICROBIALITEFORMATIONINSATONDACRATERLAKE 107 duction–methanotrophy (e.g., Berner 1971; Golubic´ 1973; Kelts and Hsu¨ waters is raised significantly, thereby lowering pHand CaCO supersatu- 3 1978;Krumbein1979;Lyonsetal.1984;ThompsonandFerris1990;Rit- ration to levels (SI (cid:53) (cid:49)0.19 to (cid:49)0.29, SI (cid:53) (cid:49)0.05 to (cid:49)0.15) Cc Arag geretal.1987;Paulletal.1992;Fortinetal.1997;Castanieretal.2000; unfavorable for CaCO precipitation (Table 1). The anoxic brines of the 3 Peckmannetal.2001).Inbiofilms,anincreaseinsupersaturationisfacil- lowermonimolimnionalsoshowaraisedsalinity,andalsoatremendously itatedbythereduceddiffusionrateswithinthemucilage. high pCO and alkalinity, so that CaCO supersaturation is raised to an 2 3 Finally, the third crucial step in biofilm calcification is the process of SI (cid:53) (cid:49)0.68 to (cid:49)0.71 and SI (cid:53) (cid:49)0.54 to (cid:49)0.57. None of the Cc Arag formation of seed crystals, which is controlled by the concentration and parameters of the carbonate system in the monimolimnion varied signifi- stereochemicalarrangementofacidicgroupsinEPS(TrichetandDe´farge cantlybetweenOctober1993andJune1996(Table1). 1995;Arpetal.1998;Arpetal.1999a;Arpetal.1999b,2001;Kawaguchi andDecho2001).Disorderedcomplexationofdivalentcations,character- MATERIAL AND METHODS isticofmanycarbohydratepolymers,shouldinhibitprecipitation.Bycon- trast,organicmatricesinbiomineralizingorganismsshowwelldefinedcar- SamplesinvestigatedinthisstudyweretakenbySCUBAdivingduring boxylategroupscorrespondingtothecrystallatticewhenattachedtosolid thedryseasoninOctober1993(Appendix1;seeAcknowledgments)and substrates and therefore promote nucleation (Addadi and Weiner 1985; shortlyafterthewetseasoninJune1996(Appendix2).103hardpartthin Mann 1988; Lowenstam and Weiner 1989; Simkiss and Wilbur 1989). sectionsof30biofilmsampleswerepreparedaccordingtomethodsinArp However,inhibitioninmanypolyanionicorganicacidsisonly temporary et al. (1998) and Arp et al. (1999a). In addition, 30 conventional thin (e.g.,Sikesetal.1994)andre-arrangementoftheacidicpolymersbyro- sectionsofdriedreef-rocksampleswerepreparedforpetrographicdescrip- tation of exocyclic groups and around the glycosidic linkages (Brant and tion.EpifluorescenceimageswereobtainedbyusingaZeissAxioplanmi- Christ1990)isassumedtoresultaccidentiallyinsuitablenucleationsites croscope equipped with a Peltier-cooled VISICAM-color CCD camera inbiofilmEPSaftersaturationwithdivalentcations(Arpetal.1999a;Arp (PCOComputerOpticsGmbH,Kehlheim)(Manzetal.2000).Imagestacks etal.1999b). withaZspacingof0.5or0.25(cid:109)mwereobtainedbyusingapiezo-mover (PhysikInstrumenteGmbH&Co,Waldbronn)attachedtoa‘‘Plan-Apoch- romat’’ 63(cid:51) objective (Zeiss, NA (cid:53) 1.4). Image processing and three- ENVIRONMENTALSETTING dimensionalrestorationwerecarriedoutbyusingtheMetamorph(cid:116)Imaging Satonda,avolcanicisland2km(cid:51)3kminsize,issituated3kmnorth software(UniversalImagingCorporation,WestChester,Pennsylvania)and ofSumbawa,Indonesia(Fig.1).Itbelongstotheinnerpartofthe6,000- theEPR(cid:121)deconvolutionsoftware(Scanalytics,Billerica,Massachusetts). km-longSundaIslandArc,whichislinkedtothesubductionzonebetween Conventionallightmicroscopywascarriedoutusingthesamemicroscope. Sumatra and the eastern Banda Sea. The island shows a central double Thechemicalcomposition(Ca,Mg,Sr,Si)ofthreesamples(including calderathatformedafterthelasteruptionmorethan4000yrB.P.(Kempe subfossil carbonates, red algal–foraminiferal crusts, recent precipitates of andKazmierczak1990a,1990b,1993;Kempeetal.1996,1997).Initially reef surfaces) was determined by electron microprobe analysis. Carbon- filled with freshwater, the crater lake was flooded with seawaterapproxi- coated polished thin sections of LR-White-embedded samples wereused. mately3000yrB.P.Today,thereisnoconnectiontothesurroundingsea. Theanalyseswereperformed at 15 kVand 12 nAon aJEOL JXA8900 Thelakelevelremains1–2mhigherthanthatofthesea,evenduringthe RLelectronmicroprobeattheInstituteofGeochemistry,Go¨ttingen.Fifty- dry season. High organic input, intense sulfate reduction, and periods of fourspotmeasurementsandfivelinescans(166spotmeasurements)were high evaporation changed the marine lake during the last few thousand performed to differentiate mineral phases. Ca, Mg, and Si were analyzed years into an alkaline meromictic lake (Kempe and Kazmierczak 1990a, for16seconds,whereasSrwasanalyzedfor30seconds.Notethatdatain 1990b,1993;Kempeetal.1996,1997). wt%(oxides)refertowhole-rockcomposition,whereasmole%(CaCO , 3 Water-chemistry data are available for the dry season of October1993 MgCO ) and ppm (Sr) refer to the carbonate phase. The detection limit 3 and the end of the wet season of June 1996 (Table 1). The lake level (limitofquantification)isgivenbyI (cid:53)t (P;f)(cid:51)(cid:115) ,wheret (cid:53)level dl z BG z fluctuatesinarangeof1mbetweentheseasons.Inprinciple,thelakeis ofsignificance,P(cid:53)confidencelevel(95%),f(cid:53)degreesoffreedom,and divided by two chemoclines into an oxygenated mixolimnion, an anoxic (cid:115) (cid:53) standard deviation of the background intensity. Typical detection BG uppermonimolimnion(‘‘concentratedlayer’’)andananoxiclowermoni- limits are 0.06 wt % for CaO, 0.05 wt % for MgO, 0.08 wt % for SrO, molimnion(‘‘brine’’).Furtherdetailedwater-chemistrydataarepublished and 0.42 wt % forSiO . The statisticalerrorwascalculatedby(cid:68)n%(cid:53) 2 inKempeandKazmierczak(1993)andKempeetal.(1996,1997).Bicar- ((cid:207)n/n) (cid:51) 100, where n denotes the absolute counts. Typical statistical bonateproductionofthemonimolimnionsulfatereductionispartlytrans- errors are 0.17 wt % for CaO, 0.03 wt % for MgO, 0.02 wt % for SrO, ferredtothemixolimnion,raisingalkalinityto4.04–4.15meqL(cid:50)1andpH and 0.01 wt % for SiO . The locations of measurement points werecon- 2 to8.6(KempeandKazmierczak1993).Asaconsequence,supersaturation trolled by epifluorescence microscopy. The craters in the samples caused ofsurfacewaterswithrespecttocalciumcarbonatemineralsishigh(Table bytheelectronbeamwere10–15(cid:109)minsize. 1;SI (cid:53)(cid:49)0.92to(cid:49)1.00,SI (cid:53)(cid:49)0.78to(cid:49)0.86)comparedtostan- Hydrochemical calculations of saturation indices and modeling simula- Cc Arag dard seawater (SI (cid:53) (cid:49)0.76, SI (cid:53) (cid:49)0.61; Nordstrom et al. 1979). tions of seasonal lake cycle and EPS degradation were carried out using Cc Arag The salinity of the mixolimnion is 31.4 ‰ in the dry season, and drops thecomputerprogramPHREEQC(Parkhurst1995).Formass-balancecal- slightlyto29.4‰attheendoftherainyseason(Table1).Seasonalrain culations,thevolumesoflakewaterlayersweredeterminedbyareamea- precipitationlowerspH,alkalinity,andCa2(cid:49)ofthesurfacewatersatless surements (Metamorph(cid:116) Imaging software) of the Satonda crater lake than1mdepth,buttheresultingCaCO supersaturationisstillhigh(SI bathymetric map published in KempeandKazmierczak(1993).Reefsur- 3 Cc (cid:53)(cid:49)0.92,SI (cid:53) (cid:49)0.78)andonly slightlylowercomparedtothedry facebiofilmareahasbeendeterminedbyaddingaverticalcylindricalplane Arag season.TheMg2(cid:49)/Ca2(cid:49)molarratioofmixolimnionwatersvariesaround (corresponding to 0.3–0.9 m depth) to the horizontal area of flooded reef 10,thusfavoringcalciumcarbonatetoprecipitateasaragonite.Withregard tops, followed by multiplication by a roughness factor. The latter factor topCO ,themixolimnionwatersareslightlyundersaturatedasaresultof has been calculated from surface morphology of reef-top thin sections. 2 algal photosynthesis, above all by the extensive green algal carpet. Only Biofilmvolumeofreeftopsectionwasdeterminedfrombiofilmthickness the surface water pCO (0.5 m depth) of the rainy season is almost in (140 measurements in 7 thin sections) and reef surface area as described 2 equilibriumwiththeatmosphere(Table1). above. Thecalcifiedproportionofbiofilmswasascertainedin7thinsec- The‘‘concentratedlayer’’ofthemonimolimnionshowsaraisedsalinity, tions from total biofilm area and calcified area in thin sections using the whichisconstantbetweentheseasons(Table1).ThepCO oftheanoxic Metamorph(cid:116)Imagingsoftware. 2 108 G.ARPETAL. RESULTS Electron microprobe analyses (Fig. 4) revealed that fibrous crust parts consist of aragonite with 98.4 mole % CaCO and 8400 ppm Sr (i.e.,55 3 FaciesSuccessionofSubfossilReefCarbonates wt%CaO,1wt%SrO),whereascryptocrystallinetoamorphouspartsare composedofanundeterminedphasewithan(cid:59)1:1molarratioofMgand Nocomplete,continuoussectionthroughthered-algal-microbialitereefs Si (36–45 wt % SiO , 15–29 wt % MgO). No high-Mg calcite has been isavailable.Maximumtotalthicknessfromthevolcanicsubstraterockto 2 detectedwithinthemicrostromatoliticcrusts. thelivingsurfaceisestimatedtobeapproximately1m(KempeandKaz- Numerous organic remains are enclosed within the microstromatolitic mierczak1993).Asuccessionofthreemajorfaciestypesisreconstructed crusts. Most striking are abundant, straight to curved filaments 1 (cid:109)m in on the basis of blocks broken from the reefs. The contact with basement diameter and up to more than 200 (cid:109)m in length, which appear dark in boulderswasobservedonlyatthesubaeriallyexposedreeftopsduringthe transmitted light (Fig. 2B–D). They cross-cut the fibrous aragonite fabric dryseasoninOctober1993. andshowoccasional,irregularbranching.Farlessabundantarebrownish, Inprinciple,thebaseofsuccessioniscomposedofaserpulidtubeframe- organic-walledspheres5(cid:109)mindiameterthatoccurisolatedoringroups work(‘‘serpulite’’).Themajorpartofthereefisformedbyamicrobialite of three to more than ten (Fig. 2E). Although enclosed in the aragonite, encasing green-algal molds. This facies type is the dominant microbialite there is no interference with the fibrous crystallite fabric. Aggregates of portion of the reefs and was studied in greater detail. The youngest car- brownish,coccoidremainsthatmightrepresentformercoccoidcyanobac- bonate veneers are formed by red-algal crusts with a thin living layer of teria have been observed in only a few cases and are restricted to the red algae on top, covered by living biofilms, green algae, and sponges. contactbetweenthefibrousaragonitelayersandthesucceedingamorphous Detaileddescriptionsofthethreefaciesfollow. Mg–Si layer (Fig. 2D). They occur in depressions of the fibrous layer ‘‘Serpulite.’’—This facies type was observed directly covering basalt below,buttherelativetimesequenceofthedifferentlayersandthecoccoid boulders and is also known from pit sections between the reef heads microfossilsremainsunclear.Inanycase,thethin,amorphousMg–Silay- (Kempe and Kazmierczak 1993; Kempe et al. 1996). The highly porous ers show a sharp contact with the fibrous aragonite layers below. This framework consists of serpulid tubes (250 (cid:109)m–1.8 mm inner diameter), contactcommonlyshowsscallopedmorphologies,whichareconsideredto which are encrusted by smaller coiled Spirorbis tubes. Open voids are representdissolutionpits(Fig.2B).Additionalorganicinclusionsthatoccur partly filled with volcanicdetritus(feldspar,augite),foraminifera(mainly within the aragonite include a few, single, boat-shaped diatoms less than Miliolidae) and small gastropods. Fibrous aragonite cement of varying 20(cid:109)mlong(Fig.2B).Theremainingvoidsbetweenthegreen-algal-stro- thickness (10 to 250 (cid:109)m) locally occurs inside the tubes, predominantly matolitic framework are partially to completely filled with micropeloidal inthesmallerones.Aspatialinterfingeringwiththeoverlying‘‘green-algal sediment with abundant pellets, skeletal detritus, and siliciclasticdetritus, microbialite,’’ as indicated by fibrous microstromatolitic crusts upon ser- cementedbyanamorphousmatrixorfibrousaragonite(Figs.2A,3A,4). pulitetubesandbymicropeloidalvoidsediment,wasobservedinonethin Electronmicroprobeanalyses(twolinesections:53pointmeasurements) section. The depth range of the serpulite facies is unknown. Marine bi- reveal that micropeloids and fibrous cements of the voids consist of ara- valves (Pteroidea) with Spirorbis tubes from soft sediments between the gonite. Cryptocrystalline to amorphous matrix parts are composed of an reefs at 15 m depth probably correspond to the serpulite. The serpulite undeterminedMg–Siphaseidenticaltotheonementionedabove.Incon- facieshasbeenconsideredbyKempeetal.(1996)asamarineinterstage, trast, pellets show a high-Mg calcite composition with (cid:59) 23 mole % possiblycausedbythepercolationofseawaterthroughthecraterwalldur- MgCO . ingapasthighsea-levelstage. Man3ymicrobialitesamplestakenfromthesurfaceoftheseasonallyex- ‘‘Green-Algal Microbialite.’’—This facies type corresponds to the posedreeftopsshowpoorlydevelopedmicrostromatoliticcrustsveneering ‘‘stromatolitic-siphonocladalean’’andthe‘‘peloidal’’zoneofKempeand green algal filament casts (Fig. 3A). Instead, a micropeloidal framework Kazmierczak (1993), because these two zones grade into each other ver- composed of irregular, fibrous aragonite aggregates is developed. Arago- ticallyandlaterally.The‘‘green-algalmicrobialite’’overliesthepreviously nite-cemented casts of siphonocladalean algae without stromatolitic en- described,older‘‘serpulite’’withoutsharpboundaryandisatleast20cm crustationoccurinreef-topsamples,whereasgreen-algalmoldsareabsent thick.KempeandKazmierczak(1993)reportamaximumthicknessof60– atgreaterdepths(15m).Somereef-topsamplesshowaragonite-cemented 80 cm.‘‘Stromatolitic-siphonocladalean’’samples(Fig.2A–E)havebeen accumulations of diatoms between the green-algalmolds(Fig.5A).Elec- collected only from the reef tops, whereas samples of ‘‘micropeloidal’’ tronmicroprobeanalysesindicatethatthecommonlybrownish-coloredcell limestone(Fig.3A–C)arepresentfromthereeftopdownto15mdepth. wallsofthegreenalgaearepermineralizedbyaMg–Siphase,thoughthe The basic framework of this facies is formed by tufts and bushes of microprobesamplingareacoveredthe2-(cid:109)m-thickcellwallsandadjacent erect, locally entangled tubes of siphonocladalean green algae (Figs. 2A, aragonite (Fig. 5B, C). On the basis of this observation, a Mg–Si permi- 3A).Thesearepreservedasmolds100to200(cid:109)mindiameter,eitheropen neralizationofcellwallsispossiblyanexplanationforthepreservationof orpartlytocompletelyfilledbyisopachoustobotryoidalfibrousaragonite. coccoid cell remains (Fig. 2B, D–E) in subfossil microbialites of Lake Thecell wallsand boundariesareevident at thebasalcontactsofthece- Satonda. ments. In addition, some tufts show constrictions at cell boundaries. Di- Aggregatesofaragonitemicropeloidsarecommonly25–100(cid:109)minsize chotomousbranchingisobservedrarelytoabundantlyinthedifferenttufts. and show dark microcrystalline centers with radiating, light, aragonitefi- The outer surface of the green algal filament molds is given by fibrous bers(Fig.3A).Locally,apartialsilicificationofthemicropeloidalcarbon- microstromatolitic crusts, cemented void sediment, or (in a few samples) ate(Fig.3B,C;belowred-algalcrust)preservedcoloniesofpleurocapsa- bymicrocrystallinetocryptocrystallineveneersthatareupto10(cid:109)mthick. leancyanobacteriabetweenthearagonitemicroclots.Itisnoteworthythat The characteristic microstromatolitic crusts that veneer the green algal thearagoniteisnotpresentasapermineralizationofcyanobacterialcolony moldsare0.5to1.3mmthickandarecomposedofupto20fibrouslayers, sheathsbutoccursintheformofseparatemicroclots(Fig.3C)asobserved each20to60(cid:109)mthick(Fig.2A–D).Eachlayerstartswithacryptocrys- inrecentreef-topbiofilms. tallinetoamorphousbase(oftenlessthan5(cid:109)mthick)uponwhichfibrous At present-day reef surfaces, the whole fabric is affected by younger aragonite nucleated (Fig. 2B–D). The fibrous parts are finally terminated dissolutionprocesses,whichhaveresultedinenlargementofprimaryvoids by a smooth, undulating surface. The next layer startsagain with acryp- and truncation of fabrics. The younger fibrous aragonite cements discon- tocrystalline to amorphous base. One to three of the cryptocrystalline to tinuously line the voids and smooth the microrelief. With regard to their amorphouslayersreachupto60(cid:109)mthicknessandcanbetracedthoughout fabric and chemical composition, these younger cements are identical to thecrustsofathinsection. themicrostromatolites. MICROBIALITEFORMATIONINSATONDACRATERLAKE 109 FIG.2.—Subfossilreefcarbonates.A)Mainpartofthereefcomposedofmicrostromatoliticcrusts(strom)encasingformerfilamentsofsiphonocladaleangreenalgae (green)similartotherecentCladophoropsis.Frameworkvoid(void)ispartlyfilledbymicropeloidalsediment,fecalpellets,andskeletalandsiliciclaticdetrituswithinan amorphousormicrocrystallinematrix.Dryshore,reef#1.Transmittedlight.SampleSat93/74(1813).B)High-magnificationviewofmicrostromatolitelaminaealternation. Notethesharpbasalcontact(dashedline)oftheMg–Silayer.Theassociatedpits(pit)arecuttingintothefibrousaragonite,probablyindicatingdissolutionpriortoor concurrentwiththeformationoftheMg–Si-phase.Noteelongateddiatomremains(dia)withinthetopofthefibrousaragonitelayer.Dryshore,reef#1.Transmittedlight. SampleSat93/74(1813).C)Microstromatoliticcrust(strom)composedoffibrousaragonitelayers(light)andthinlaminaeofanamorphoustomicrocrystallineMg–Si phase(dark).Totheleft,thebasalcontacttoaformergreen-algalfilament(green)isvisible.Dryshore,reef#1.Transmittedlight.SampleSat93/74(1813).D)Detailof partCshowingfilamentousstructures(fil)ofsupposedfungalorigincross-cuttingthefibrousaragonitefabricofthemicrostromatolites.Thefilamentousstructuresare interpretedtobeofendolithicorigin,thereforeshouldbedestructiveratherthaninvolvedinconstructiveprocessesofcrustformation.Noteremainsofcoccoidmicroor- ganisms (cocc) within the top of one of the aragonite layers. Such coccoid remainsare rare and mightresult form coccoid cyanobacteria,althoughtheirroleincrust formationremainsinterpretive.Dryshore,reef#1.Transmittedlight.SampleSat93/74(1813).E)Remainsofcoccoidmicroorganisms(cocc)withinthebasalpartofa fibrousaragonitelayer.Thesespheresmayresultfromcoccoidgreenalgae,cyanobacteria,orspores.Becauseofthescatteredarrangementoftheremains,anoriginfrom benthiccoccoidcyanobacteriaisconsideredunlikely.Thelarge‘‘sphere’’isanartificialbubbleinthesection.Dryshore,reef#1.Transmittedlight.SampleSat93/74 (1813). 110 G.ARPETAL. FIG.3.—Subfossilreefcarbonates.A)Green-algal-microstromatoliteframestoneshowingerectgreen-algalfilamenttubes(green)encrustedbymicrostromatolites(strom), and voids filled with aragonitic micropeloids (pel) within a Mg–Si matrix. Note fan-shaped aragonite cements (cem) that formed within voids. 0.3 m depth, reef #1. Transmittedlight.SampleSat96/14.B)Micropeloidalaragonitelayer(microclots),containingsilicifiedpleurocapsaleancyanobacteria,overlainbyared-algalcrust(pey). 7mdepth,reef#1.Transmittedlight.SampleSat93/6.C)Silicifiedpleurocapsaleancolonies(pleu)betweenaragonitemicroclots(arag)ofthemicropeloidallayershown inpartB.7mdepth,reef#1.Transmittedlight.SampleSat93/6. Conspicuousstructuresassociatedwithmicrostromatoliticcrustsandmi- crusts are missing. Electron microprobe analyses (Appendix 3, see Ac- cropeloidal parts in semicryptic voids of Satonda reefs are semiglobular knowledgments) indicate an aragonite mineralogy for Peyssonnelia thalli structures (Fig. 6A, B), which have been compared to the Paleozoic mi- andfibrouscements,high-Mgcalciteforfecalpellets,micritefillingswithin croproblematicum Wetheredella (Kazmierczak and Kempe 1992). The Peyssonneliathalli,andnubecullinidforaminifera,andamorphousMg–Si semicircular to halfmoon-shaped sections are 90–190 (cid:109)m in height and formatrixparts. 120–290(cid:109)minwidth.Oneortwoaragonitearrays,whichoriginateatthe Afinal,dense,smoothtodendroidcrustcomposedofthesquamariacean basalsubstrate,formthesestructures.Theouterlimitissharplydefinedby Peyssonnelia, the coralline red alga Lithoporella, and nubecullariid fora- adarklineoralessthan5(cid:109)mthickorganicwall(Fig.6B). miniferaformovergrowthsonthecornflake-likered-algalcrustsandolder ‘‘Red-Algal–Foraminiferal Crusts.’’—Red-algal–foraminiferal crusts microbialites (Fig. 3B). The Peyssonnelia thalli commonly show a light– formtheyoungestpartsoftheSatondareefs.Theyveneerolderreefparts darklaminationduetoalternatingcellsizes.Layersofsmallcellsthereby fromtheseasonallowstandleveldowntobelowthechemoclineat22–24 appear ‘‘micritic’’ at first glance but are composed of aragonitic microfi- mdepthandcorrespondtothe‘‘cyanobacterial–red-algalzone’’ofKempe bers. Mineralized coccoid cyanobacteria have been observed in the hori- and Kazmierczak (1993). The highly porous, cornflake-like framework is zontal crevices between the thalli. However, all of these coccoid cyano- composedoffoliaceousthalliofthesquamariaceanredalgaPeyssonnelia bacteria were preserved by silification (amorphous Mg–Si phase) and no withirregulartolenticular,millimeter-sizevoidsinbetween(Fig.6C).The CaCO permineralization was found. Electron microprobe analyses per- 3 voidsarepartiallyorcompletelyfilledbyaragonite-cementedfecalpellets, formedonthisfinaldensered-algalcrustconfirmthepreviouslydescribed miliolidforaminifera,raregastropods,andpatchesofmicropeloids.Lower compositionofskeletons(Fig.7).AragoniticPeyssonneliathallialternate sides of Peyssonnelia thalli are characterized by hypobasal aragonite bo- with high-Mg calcitic Lithoporella thalli and nubecullinid foraminiferal tryoids that are marginally micritized (Fig. 6C; Kempe and Kazmierczak tests.TheamorphousMg–Siphaseisrestrictedtohorizontalcrevicesnear 1993).Thesehighlyporouscrustsarecommonly5cm,andlocally15–25 thereefsurfaceandafewsmallporespaces,butitalsooccursasapartial cm,thickatdepthsbelow5m(KempeandKazmierczak1993).Thethick- silificationofMg-calciticLithoporellathalli(Fig.7). ness of these crusts decreases downwards to 3–4 cm close to the chem- The uppermost living thalli of the final dense red-algal crust represent ocline. In shallow reef parts (0.3 m below seasonal lowstand) red-algal the presently growing part of the reefs. In shallow water this final crust MICROBIALITEFORMATIONINSATONDACRATERLAKE 111 FIG.4.—Electronmicroprobetraverseofamicrostromatoliticcrustofthesubfossilreefcore.SampleSat93/74(1813).Dark-appearinglayersarecomposedofaragonite, whereasthin,lightlayersareformedbyanunidentifiedMg–Siphase.High-Mgcalciteisrestrictedtointernalsedimentofpocketsbetweenthemicrostromatolites. 112 G.ARPETAL. FIG.5.—Electronmicroprobesectionofasubfossilgreen-algal-microbialitesampledataseasonallyexposedreeftop.SampleSat96/18,0.5mdepth,reef#1.A)Overview of the green-algal-microbialite section. Spaces between green-algal moulds are infilled by halfmoon-shaped diatoms encased in aragonitecement. Whitelineindicates transectinpartB.B,C)Transectacrossmarginalpartsofgreen-algalmolds,theirwalls,andthecarbonatebetweentwofilaments.Measurementpoints23and32are interpretedasmixedsignalsofanamorphousMg–Siphaseandthesurroundingaragonite(noteSrcontents). locally veneers directly the ‘‘green-algal microbialites,’’ separated by a alean green algae. The squamariaceanPeyssonneliadominatesinshallow corrosion plane. Crusts dominated by nubecullariid foraminifera protrude water,whereasthecorallinaceanLithoporellaisincreasinglyabundantwith even into near-surface cavities of the subfossil cornflake-like red-algal depth.However,livingthalliofbothtaxaarepresentthoughoutthiszone frameworkandcorrodedgreen-algalmicrobialites. (Fig.8A,B).Insectlarvaltubes,whichoccurregularlyatshallowdepths, areveneeredbythered-algalthalli,also.Reef-surfacebiofilmsofthered- Reef-Surface-LivingBiotaandBiofilmsoftheDrySeason algalcrustsarediscontinuouslydeveloped,usuallylessthan10(cid:109)mthick, andcompriserod-shapedandfilamentous,non-phototrophicbacteria(Fig. Samplesfromthelivingreefsurfacewereobtainedfromthewaterline 8E,F).Heterotrophicbacteria,whichdigestcellwallsofdeadgreenalgal downtobelowthechemoclineat22mdepth(Appendix2).Owingtogaps filaments, have been detected by TEM sections (Arp et al. 1996). In ad- in the sampling profile, boundaries or transitions cannot be assigned to dition,fungalhyphae,whichpenetratefilamentsofcyanobacteria,arealso defineddepths. present(Arpetal.1996).Hadromeridsponges(Laxosuberitessp.)locally Cyanobacterial and bacterial biofilms on living red algae are generally less than 10 (cid:109)m thin and grow preferentially within depressions or sub- veneerthelivingordeadred-algalthalli.Theircontactwiththecalcareous red-algalsubstrateisalwaysmediatedbyathinbiofilmofnon-phototrophic surfacevoidsofthered-algalcrusts.Itisimportanttonotethatallinves- tigated biofilms generally comprise a large portion of non-phototrophic bacteria. microorganisms,aboveallfilamentousbacteria.Inaddition,fungalhyphae Phototrophic microorganisms, such as pennate diatoms, coccoid algae and numerous coccoid and rod-shaped bacteria are present, especially at (Fig. 8E, F; ‘‘Dermocarpella’’), and the cyanobacteria Pleurocapsa and decayingspongetissues.Threezonesbetweenthelake-levellowstandand Phormidium spp., occur only scattered on the red-algal surfaces. Only in thechemoclineandonebelowthechemoclinearedefined. depressions of the surface and at the surface of green algal filaments are Peyssonnelia–Lithoporella Zone.—Reef surfaces betweentheseasonal cyanobacteriaanddiatomsmorecommon,alwaysassociatedwithnon-pho- lowstand and approximately 7 m depth are characterized by living Peys- totrophic bacteria. In contrast, several samples showabundant pleurocap- sonnelia–Lithoporellacrustsandadensemeadowofattachedsiphonoclad- salean cyanobacteria in subsurface voids between the living crustosered- MICROBIALITEFORMATIONINSATONDACRATERLAKE 113 FIG.6.—SubfossilWetheredella-likestructuresandreefcarbonates.A)Subfossilcystousstructures(weth)formingacrustwithinacavityofthered-algalreefframework (red).Thesecystousstructures,superficiallyreminiscentofthePaleozoicmicroproblematicumWetheredella,areconsideredhereintobefossilizedrestingbodiesofsponges. Theremainingporespaceisfilledbyaragonite-cementedmicropeloids(pel).Dryshore,reef#1Transmittedlight.SampleSat93/44.B)Close-upviewofWetheredella- like structures showing radial-fibrous aragonite and a distinct, defined wall (wall). Dry shore, reef#1.Transmitted light. Sample Sat 93/44. C)Foliaceousthalliofthe calcifiedsquamariaceanredalgaPeyssonnelia(pey)forma‘‘cornflake-like’’reefframeworkintheyoungerreef.Lowersidesofthethallioccasionallyshowhypobasal aragonitebotryoids(botr).Theremainingirregulartolenticularvoidsarepartlyfilledbyaragonite-cementedmicropeloids(pel),fecalpellets(faec),andfibrousaragonite cements(cem).24–25mdepth,reef#10/11.Plane-polarizedlight.SampleSat93/28. algal thalli (Fig. 8B). In addition, endolithic cyanobacteria of the Hyella diameter)occurinvaryingabundancealso.LivingLithoporellamonolayers group, which bore in living and dead Peyssonnelia thalli, have been ob- are still present, but less abundant and discontinuously distributed within servedinonesamplefrom0.5mdepth(Fig.8C,D).Pleurocapsacolonies the biofilm. Siphonocladalean green algae have not been found at that areabundantonlyincrevicesbetweenthered-algalthalli,buttheyremain depth,buthadromeridspongesoccuratleastatupto18mdepth. softand unmineralized (Fig. 8B).Green-algalholdfastsarelocatedinthe Non-PhototrophicBiofilmZone.—Onthebasisofonesampletakenat depressions or are already overgrown by the calcareous red-algal thalli. 24–25mdepth,reefsurfacesbelowthechemoclinearecomposedofdead Suchenclosedgreen-algalfilamentsformtheonlyplaceswherenon-skel- Peyssonnelia–Lithoporella crusts that are veneered by 10–20 (cid:109)m thick, etal aragonite precipitation was observed in one single sample (Fig. 8A). detritus-rich, soft biofilms of non-phototrophicbacteria.Thedetrituscon- Fibrous aragonite formed inside of lysed green-algal cells, whereas ara- sists of organic particles, siliciclasticgrains ((cid:44) 10 (cid:109)m) and few Mnhy- gonite precipitates were observed neither in adjacent biofilms of the reef droxide particles. Microorganisms are dominated by filaments more than surfacenorincrypticbiofilmsintheirvicinity.Crevicesandvoidsofthe 60(cid:109)mlongand0.4–0.5(cid:109)mthick,ofsupposedbacterialorigin.Nobranch- red-algalframeworkarecommonlyrichinflocculentorganicmaterial(de- ing has been observed in these filaments. In addition, numerous coccoid caying sponge tissues, organic detritus) and cyanobacteria, and reveal an (0.65 (cid:109)m) and rod-shaped (0.25–0.5 (cid:109)m diameter; 1 (cid:109)m long) bacteria extensivepopulationofcoccoidbacteria,whichapparentlydecomposethe are present in the biofilm. Rare spirillae (0.5 (cid:109)m; 3.25 (cid:109)m long) and organics. No carbonateprecipitateswereobservedinassociationwiththe coccoid phototrophs (?cyanobacteria, 2.3 (cid:109)m in diameter) occurin small bacterialdecompositionoforganicmatter.Figure9isaschematicdrawing groupsofeightcellsandless. that summarizes the observations on samples from the reef-surfacecrusts ofthePeyssonnelia–Lithoporellazone. Reef-Surface-LivingBiotaandBiofilmsoftheWetSeason Pleurocapsa–‘‘Dermocarpella’’ Zone.—Deeper-water samples (14–18 m depth) have surfaces that are composed largely of dead Peyssonnelia Thezonationofthereef-surfacecommunitiesinJune1996differedfrom thalli that are overgrown by almost continuousPleurocapsa–‘‘Dermocar- thatof October1993.ThelakelevelinJune1996, afewweeksafterthe pella’’biofilms.ErectandprostratePhormidiumspp.(0.75(cid:109)mand2(cid:109)m end of the rainy season, was still approximately 0.4–0.5 m higher than 114 G.ARPETAL. FIG.7.—Electronmicroprobetraverseofared-algal-foraminiferalcrustofthereefsurfaceat17mdepth(reef#1).AragoniticPeyssonneliathallialternatewithMg- calciticLithoporellathalli.AnamorphousMg–SiphaseoccursinvoidsandcrevicesbutalsopartiallyreplacesMg-calciticLithoporellaskeletons.

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Hydrochemistry data and model calculations indicate that CO2 de- gassing after seasonal mixis conditions because of sulfate reduction in bottom sediments and pro- nounced Occasional Spirulina and Oscil- latoria filaments
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